udpscope: an oscilloscope view for a UDPStreamer source

GTK4 and libadwaita. It receives one UDP stream through the vendored
udpsclient, buffers it in a large ring, and draws it with the vendored
plotter: a trigger with a pre/post event window, a pause that freezes the
window, cursors, measurements, math channels in a node editor, an event cache,
named configurations, preferences, and an export of the visible window to
.npy/.csv. Waveform files can be opened and analysed in place.

The interface is two threads -- a producer that owns the socket and the GTK
main thread that owns every widget and the GL context -- sharing one model
under one mutex, with samples crossing through lock-free rings.

deps/plotter is a submodule pinned to the commit this tree builds against;
deps/udpsclient is vendored in place. docs/claude/ documents the rules that
cost a defect to learn, and SPECS.md is the requirement document.

docs/claude/verification.md

@@ -301,9 +301,11 @@ as distinct from the fault injection below, which exercises ingest:
   `[events] activated row0: replaying=1 thumbnail=1 thumb_size=108x36 thumb_visible=1` (the size is
   what says the picture is *on screen*: a paint-able alone is not), `[events] replayed: channels=4
   samples=20001 replaying=1 waveform=1` and `[events] live again: replaying=0 waveform=0`. It ends on
-  the pane's other precondition, through the trigger panel's switch: `[events] recording off, trigger
-  off` and `[events] recording off, trigger on` must **both** read `button_visible=0 pane=0
-  recording=0` -- the pane goes with its switch, in either trigger state. **`samples` must equal the
+  the pane's availability, through the trigger panel's switch: `[events] recording off, trigger off` and
+  `[events] recording off, trigger on` -- once the cache holds events (the probe's own run has written
+  some by this point) both read `button_visible=1 pane=1 recording=0`, because a button that shows the
+  cache must not go away with the switch that wrote it; against an *empty* cache the same two lines read
+  `button_visible=0 pane=0`, which is the empty column the item exists to prevent. **`samples` must equal the
   export's row count for the same window** (`20001` for a 2 s event at 10 kS/s), `thumb_bytes` must be
   a real BMP (`54 + 480*160*3`-ish, i.e. hundreds of kB, not 0), `thumbnail` must be 1 (the row loaded
   the picture) and the last line must read `replaying=0` -- a replay with no way back is a one-way